There are various forms of flail-type cutters being marketed for the purpose of performing cutting and similar functions such as light cultivation and edging by means of non-rigid filaments which extend generally radially outwardly from a cutter head rotated at high speeds. Desirable results are obtained by these filaments rotating at high speed under which they assume a tensioned condition due to centrifugal force. It has been found that these filaments can be used to cut grass and other delicate vegetation more safely and with a greater degree of flexibility than devices using rotating rigid blades.
The industry producing cutters of this type has generally avoided the use of either metallic or composite filaments inasmuch as it has seemed that filaments of this type exhibit inferior operating life characteristics when used with commonly available engines and electric motors. In particular, the metallic or composite filaments have been found to break more rapidly than plastic filaments.
However, I have found that the operating life characteristic of filements, whether plastic, metallic or composite, is more directly a function of the system employed to rotate and restrain the free end of the filement than a function of the filament material. Furthermore, prior devices have not addressed the fundamental problem of flail cutting which will be herein described. The disclosure of the nature and effect of several basic phenomena which occur and whose effects are interrelated, the statement of the basic problem, and the disclosure of a system which deals with that problem so as to accommodate and control the basic phenomena, will yield significant advances in the state of the art.
Hereinafter will be described the basic phenomena, the basic problem and a radically new, different and more productive search to the solution of the basic problem to yield substantial advancement of the art and enable the production of a new generation of filament cutters with characteristics far superior to those now available.
In flail cutting a flexible filement is rotated at some angular velocity. The centrifugal force of rotation imposes an axial load on the filament. Each segment of the filament has a kinetic energy which is EQU KE = 1/2 mv.sup.2
where
m = mass of the segment PA1 v = velocity of the segment PA1 r = distance from center of rotation PA1 w = speed of rotation PA1 m = mass PA1 w = speed of rotation PA1 r = distance from center of rotation
The velocity of each segment is a function of its distance from the center of rotation or EQU v = rw
where
Therefore the kinetic energy of a segment is EQU KE = 1/2 m (rw).sup.2
This kinetic energy is available for instantaneous release when the filament encounters an obstacle such as a blade of grass. It is this eneregy release which effects cutting and other action which can be achieved in flail cutting.
Thus energy is stored in the filament and alternately released through impact and replenished by the power source, typically an electric motor or gasoline engine.
Cutting action improves as the ratio of stored kinetic energy to filament diameter is increased. This can be seen intuitively since for a given filament diameter a greater energy release will provide more effective cutting action.
Thus it would seem that higher rotational speed would be desirable. However, rotational speed also imposes a centrifugal load on the filament which is EQU C = mw.sup.2 r
where
Thus each filament segment exerts a force on every other segment nearer the center of rotation or point of restraint. Above a threshold speed, the summation of the forces acting on the extended filament yields a total force at the point of restraint such that at that point the tensile strength of the filament is exceeded producing failure.
In flail cutting the filament does not assume a straight radial position relative to the axis of rotation of the cutter head (even when not encountering obstructions) inasmuch as wind force acting upon the extended end portion of the filament imparts a drag thereon. Thus, the extended end portion of the filament continuously curves rearward relative to the direction of rotation. As the filament encounters obstructions, phenomena occur which coact to shorten the operating life of the filament. These phenomena will be called impact shear, impact abrasion, impact heating and tensile fatigue.
When a segment of the filament, traveling at high velocity, encounters an obstacle, the inertia of the free end of the filament and the inertia of the obstacle encountered acting in opposition to the inertia of the filament segment causes the filament to partially "wrap around" the obstacle. During this "wrap around" period the local filament segment experiences shear forces whose magnitude are a function of the contour of the obstruction, the mass of the obstruction, the velocity of the filament and the cross sectional mass and geometry of the filament. If the obstacle encountered has sufficient mass the filament will fail at the point of contact with the obstacle or at some adjacent point if there is a weakness in the filament. Impact shear occurs with every impact so that the filament encounters shear loading at very high frequency. Even where the individual impact does not produce instaneous failure, it induces localized damage making the filament more susceptible to future failure at that point. Thus, impact shear has an accumulative effect which is an important factor in the operating life expectancy of a given segment of the filament. Also, because the kinetic energy of a given segment increases with the square of its distance from the center of rotation, the outermost segments of the filament are most susceptible to failure in a shear mode.
Filament abrasion occurs when an obstruction has sufficient mass and geometry to actually cut away a portion of the filament. Thus, the filament may be "niched" or split at a given point but may not, at that instant, totally sever. This form of damage typically occurs when the filament encounters very dense obstacles such as rocks or curbing, asphalt and similar physical features and also has an accumulative effect in shortening the operating life of a filament segment. In the case of plastic filaments, this phenomenon frequently results in the "split end" condition where the tip of the filament splits into several small fibers. Of course, a plurality of smaller fibers, due to their greater flexibility and reduced inertia do not operate as efficiently in cutting vegetation. Many metallic and composite filaments exhibit shear and abrasion resistance superior to that obtainable from plastic filaments. Accordingly, metallic and composite fibers are less susceptible to failure due to abrasion and shear.
Impact heating is generated by the rubbing action which occurs when a filament section impacts with an obstruction and by hysteresis as the filament undergoes deformation as a result of impact. In general, insufficient heat is generated to melt a plastic filament segment at the point of impact, although heating as a result of impact becomes a more serious problem as larger diameter filaments are used.
It is obvious that an individual filament encounters a very high number of impacts per second. This induces very high frequency vibrations in the filament. Plastic filaments do not transmit vibrations as well as metallic filaments and a smaller percentage of the overall tensile load is transmitted to the point of restraint. The most severe tensile load on the filament occurs at the point where it is constrained. With conventional cutting heads this is either at the opening in the housing or hub or inside the housing or hub on the spool upon which the filament is wound, or a combination of the two.
Plastic, metallic and composite filaments are all subject to tensile failure at the point of constraint. Since the fatigue of the filament is a combined function of its angular velocity, its extended length and mass, the loading it encounters from impact with obstructions, and the structure by which it is restrained, if fatigue of the filament is suitably controlled, filaments utilized for cutting purposes can be employed with superior results.
Acceptance of the prior assertion requires a statement of the fundamental problem of flail cutting in the context of the present state of the art.
The present state of the art is such that filament breakage is believed to be the fundamental problem. This is evidenced by the attention devoted to minimizing filament breakage. It has been recognized that breakage often occurs at the point of primary filament restraint which is typically at the exit opening in the cover which houses one or more filament reservoirs. Various devices have been employed, such as the "curvilinear bearing surfaces" generally described in U.S. Pat. Nos. 3,708,967, 3,826,068 and 3,859,776, to reduce the incidence of such breakage.
However, it is asserted that filament breakage per se is not the fundamental problem and that optimum system performance will result when the total breakage rate is sacrificed to obtain preferential breakage which will be hereinafter explained.
The fundamental problem, with the present state of the art, is that filament breakage causes inconvenience and greatly reduces the productivity of the cutting system. This stems from the necessity of stopping the rotating filament reservoir in order to activate the various mechanisms which have been devices to enable additional filament to be extended. There is an inherent attendant drawback in that the user becomes accustomed to frequent physical contact with portions of the cutting system which are capable of imparting injury.
The user approaches physical contact with a rigid steel lawnmower or edger blade with caution on the relatively infrequent occasions when it is necessary. However, presently available flail cutting systems require such contact with a frequency which tends to invite carelessness.
In addition, the inconvenience inherent in present filament storage and feed systems inhibits the user in applying the systems to heavy growth and almost totally precludes their use in other applications, such as lawnmowers, where it would be impractical to advance additional filament using present feed systems.
Thus, it is asserted that the basic problem is not one of filament breakage. The basic problem is to provide a system which controls filament breakage while providing a safe and effective means for dispensing additional filament from the reservoir to replenish segments which are lost.
The desirability of an improved dispensing system, particularly one which can be activated and controlled while the device is in continuous operation, should be obvious. The desirable elements of controlled breakage are as follows.
The filament is an expendable element of the system. However, there are clearly preferred breakage modes. Since the tip segment effects the greatest cutting action, it is abraided and worn away most rapidly. Therefore, the most preferred breakage area is near the end of the extended fiber. Conversely, the least preferred areas are those segments near the point of restraint, or worse, beyond the point of restraint into the storage area. This should be obvious since, if the filament breaks farther from the tip of the cutting end, a greater portion is lost with no commensurate benefit. Loss of short segments at the tip is much preferred.
There are three approaches which can be used to reduce breakage at the root of the filament. These approaches are reduced operating speed (which reduces the fundamental vibratory frequency and the tensile load), distribution of restraint forces over greater filament lengths (to reduce load concentration), or special restraint of the filament in such a manner as to control the fundamental "breaking" process.
The reduction of speed tends to reduce the cutting effectiveness because such effectiveness is a direct function of the kinetic energy stored in a segment of the filament. Therefore, lowering the rotational speed is not a desirable approach to reducing filament breakage, because overall performance of the system suffers.
Much has been attributed to the importance of "curvilinear" bearing surfaces parallel to the axis of rotation at the point of restraint of the filament. These "curvilinear" bearing surfaces have been provided as a means for distributing loading over a greater filament length so as to thereby extend operating life of the filament. Curvilinear surfaces are disclosed in U.S. Pat. Nos. 3,708,967, 3,826,068 and 3,859,776. However, the precise contour and function of these "curvilinear" bearing surfaces have not been defined nor has the effect of their particular contour been precisely explained. The utilization of "curvilinear" bearing surfaces has apparently been adopted as a result of observation that a sharp corner at the bearing surface results in rapid filament breakage at the point of restraint. This would be expected due to excess shear and vibratory load concentrations on the filament at that point.
While the use of a "curvilinear" bearing surface eliminates the rapid filament breakage which occurs when a sharp angle at the bearing surface is utilized, it still does not actively contribute to selective breakage outwardly of the bearing surface. When breakage of the filament occurs at the bearing surface the filament end will often be retracted within the housing surrounding the spool and it is therefore extremely difficult to extend the desired length of filament from the spool and requires access to the interior of the housing in order to thread the end of the filament to be extended through the opening in the housing for the spool defining the bearing surface.
However, if a bearing surface which causes controlled angular deformation of the filament at the bearing surface is provided and structure enabling selective increment feed of the filament from the spool during operation of the associated cutter is provided, each time a new section of filament is engaged with the bearing surface that section is subject to tensile and vibratory loading and is therefore partially weakened. Repeated increment feeding of the filament during operation of the cutter thereby produces a free end portion of the filament which has longitudinally spaced weakened zones therealong. These longitudinally spaced weakened zones are, as well as intermediate portions of the filament, subject to impact with the material being cut and impact shear and abrasion and friction heating as a result thereof. These phenomena tend to further weaken the spaced pre-weakened zones more rapidly than the "virgin" filament therebetween and the outermost weakened zone is of course more severely weakened than the preweakened zones spaced inwardly thereof. This results in a substantial majority of the breakage of the filament as a result of its use in cutting vegetation occurring at the outer end portion of the free end thereof. Therefore, inasmuch as the filament should be considered as an expendable item, a cutter including means for increment feed of the filament during operation of the cutter and also means for pre-weakening the filament in the zones thereof successively with the bearing surface of the housing is operable in a substantially continuous manner through repeated increment feeding of the filament in order to renew the outer end portions thereof which are repeatedly broken therefrom. During operation of the cutter, as successive segments of filament are broken from the free end thereof and the filament is fed in increments from the spool, the localized weakened zones disposed outwardly of the bearing surface experience cumulative stress concentration and their tensile properties degenerate. Thus, a weakened zone toward the free end of the filament, having experienced more impact with the vegetation being cut than weakened zones closer to the bearing surface, is weaker and more likely to sever. This yields selective severing or breakage of the filament with a higher incidence of breakage near the tip of the filament. Therefore, increment feeding of the filament may be continued during operation of the cutter with a substantial reduction of unwanted breakage of the filament at the bearing surface or inwardly thereof necessitating operation of the cutter to be terminated in order to manually extend a new length of filament from the housing.
The preferred geometry of the bearing surface is a function of the filament geometry, material properties, the extended filament length and the average speed of rotation. However, it has been determined that for various available metallic, non-metallic and composite filaments, and for speeds of rotation between 1000 and 10,000 rpm a bearing surface with at least one angular break where the included angle is not greater than 178.degree. nor less than 100.degree. will produce localized filament compression and weakening to achieve the desired result of selective breakage. Where multiple angular surfaces are employed they may be separated by straight or curved surface segments without noticeable difference but the distance between pairs of relatively angulated surfaces should be substantially less than the length of the increment of filament extended by one advance of the feed mechanism so as to insure that the extended segment of the filament has discrete weakened zones.